Method for solid oxide fuel cell anode preparation

ABSTRACT

A method for preparation of an anode for a solid oxide fuel cell in which a plurality of zircon fibers are mixed with a yttria-stabilized zirconia (YSZ) powder, forming a fiber/powder mixture. The fiber/powder mixture is formed into a porous YSZ layer and cacined. The calcined porous YSZ layer is then impregnated with a metal-containing salt solution. Preferred metals are Cu and Ni.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for preparation of anodes for use insolid oxide fuel cells. More particularly, this invention relates to amethod for preparation of an anode for a solid oxide fuel cell in whichmetals and catalytic materials employed in such anodes are added in aseparate step compared to conventional methods of anode preparation.

2. Description of Prior Art

Solid oxide fuel cells have grown in recognition as a viable hightemperature fuel cell technology. There is no liquid electrolyte withits attending metal corrosion and electrolyte management problems.Rather, the electrolyte of the cells is made primarily from solidceramic materials so as to survive the high temperature environment. Theoperating temperature of greater than about 600° C. allows internalreforming, promotes rapid kinetics with non-precious materials, andproduces high quality by-product heat for cogeneration or for use in abottoming cycle. The high temperature of the solid oxide fuel cell,however, places stringent requirements on its materials. Because of thehigh operating temperatures of conventional solid oxide fuel cells(approximately 1000° C.), the materials used in the cell components arelimited by chemical stability in oxidizing and reducing environments,chemical stability of contacting materials, conductivity, andthermomechanical compatibility.

The most common anode materials for solid oxide fuel cells are nickel(Ni)-cermets prepared by high-temperature calcination of NiO andyttria-stabilized zirconia (YSZ) powders. High-temperature calcinationis essential in order to obtain the necessary ionic conductivity in theYSZ. These Ni-cermets perform well for hydrogen (H₂) fuels and allowinternal steam reforming of hydrocarbons if there is sufficient water inthe feed to the anode. Because Ni catalyzes the formation of graphitefibers in dry methane, it is necessary to operate anodes atsteam/methane ratios greater than 3. However, there are significantadvantages to be gained by operating under dry conditions. Progress inthis area has been made using an entirely different type of anode,either based on ceria (See Eguchi, K, et al., Solid State Ionics, 52,165 (1992); Mogensen, G., Journal of the Electrochemical Society, 141,2122 (1994); and Putna, E. S., et al., Langmuir, 11 4832 (1995)) orperovskite anodes (See Baker, R. T., et al., Solid State Ionics, 72, 328(1994); Asano, K., et al., Journal of the Electrochemical Society, 142,3241 (1995); and Hiei, Y., et al., Solid State Ionics, 86-88, 1267(1996).). These oxides do not, however, provide sufficient electronicconductivity. Replacement of Ni for other metals, including Co (SeeSammes, N. M., et al., Journal of Materials Science, 31, 6060 (1996)),Fe (See Bartholomew, C. H., Catalysis Review-Scientific Engineering, 24,67 (1982)), Ag or Mn (See Kawada, T., et al., Solid State Tonics, 53-56,418 (1992)) has been considered; however, with the possible exception ofAg, these are likely to react with hydrocarbons in a way similar to thatof Ni. Substitution of Ni with Cu would also be promising but for thefact that CuO melts at the calcination temperatures which are necessaryfor establishing the YSZ matrix in the anodes.

It is also well known that the addition of ceria to the anode improvesperformance. However, the high-temperature calcination utilized inconventional anode preparation causes ceria to react with YSZ, as aresult of which performance is not enhanced to the extent which could bepossible if formation of ceria-zirconia did not occur.

SUMMARY OF THE INVENTION

Accordingly, it is one object of this invention to provide a method forpreparation of solid oxide fuel cell anodes which enables the use oflower melting temperature materials than employed by conventional solidoxide fuel cell anodes.

It is another object of this invention to provide a process for solidoxide fuel cell anode preparation which enables efficient operationusing dry natural gas as a fuel.

It is yet another object of this invention to provide a method for asolid oxide fuel cell anode preparation which enables the use of ceriato improve anode performance while avoiding the formation ofceria-zirconia which reduces the extent of performance enhancement inconventional solid oxide fuel cell anodes.

These and other objects of this invention are addressed by a method forpreparation of an anode for a solid oxide fuel cell in which a pluralityof zircon fibers or other porous matrix material is mixed with ayttria-stabilized-zirconia (YSZ) powder, thereby forming a fiber/powdermixture. The fiber/powder mixture is then formed into a porous YSZ layerand calcined. The calcined porous YSZ layer is then impregnated with ametal-containing salt solution. Accordingly, contrary to conventionalmethods for solid oxide fuel cell anode preparation, the method of thisinvention results in a YSZ layer which remains highly porous followinghigh-temperature calcination to which any suitable metal, including Cuand Ni is then added by impregnation of the salt solution, after thehigh temperature calcination of the YSZ layer. In addition to enablingthe use of metals whose oxides have a low melting temperature, themethod of this invention also allows catalytic materials, such as ceriaand/or palladium (Pd) to be added in controlled amounts in a separatestep.

Cells prepared in accordance with the method of this invention with Niperform in a very similar manner to those cells prepared usingconventional means. With Cu used in place of Ni, there is a possibilityof oxidizing hydrocarbons directly, particularly since Cu is inert indry methane. Even without direct conversion, the Cu-YSZ anode allows theuse of dryer gases (partially reformed methane), because Cu is inert tomethane. To convert methane, it is necessary to add a catalyticcomponent. Ceria, particularly when doped with noble metals like Pd, Pt,or Rh, is active for this process.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of this invention will be betterunderstood from the following detailed description taken in conjunctionwith the drawings wherein:

FIG. 1 is a diagram showing the I-V relationship for cells at 800° C. inH₂ prepared by impregnating porous YSZ with 40% Cu in accordance withone embodiment of this invention;

FIGS. 2A, 2B, and 2C show SEM micrographs for conventional Ni-cermetprepared from NiO and dense yttria-stabilized zirconia, designated asYSZd, neat porous yttria-stabilized zirconia designated as YSZp, andCu-cermet prepared from 25% YSZd impregnated with 40% Cu, respectively;

FIG. 3 is a diagram showing the I-V relationship for cells at 800° C. inH₂ prepared by impregnating porous YSZ (25% YSZd) with varying amountsof Cu;

FIG. 4 is a diagram showing the I-V relationship for cells at 800° C. inH₂ with a conventional Ni-cermet anode, a Ni-cermet anode prepared fromporous YSZ (25% YSZd), and Cu-cermet prepared from porous YSZ (25%YSZd);

FIG. 5 is a diagram showing the I-V relationship for cells at 800° C. inH₂ with the addition of ceria to Cu/YSZp;

FIG. 6 is a diagram showing the I-V relationship for cells at 800° C. inH₂ with the addition of ceria to Cu/YSZ mixture;

FIG. 7 is a diagram showing current density as a function of time forNi-and Cu-based cells during switching of fuels from dry H₂ to dry CH₄and back; and

FIG. 8 is a diagram showing the I-V relationship for Cu-based cells withmethane.

DESCRIPTION OF PREFERRED EMBODIMENTS

As previously stated, conventional solid oxide fuel cells are unable tooperate efficiently in dry natural gas. This is due to the fact thatthey typically have Ni-cermet anodes which are prepared byhigh-temperature calcination of powders consisting of NiO andyttria-stabilized zirconia. High-temperature calcination is essential inorder to obtain the necessary ionic conductivity in the YSZ. Under thereducing conditions of operation, NiO is reduced to the metal andprovides electronic conductivity. However, in dry methane, Ni tends toform graphite fibers which quickly deactivate the system. Ni can bereplaced by Fe or Co, but these metals suffer from similar problems. Wehave developed a method for preparation of an anode for a solid oxidefuel cell which allows the addition of the electron-conducting metal,including metals like Cu, for which the oxide melts at low temperature,in a manner which does not require the high-temperature calcination ofconventional methods. In addition, catalytic and ion-transfercomponents, such as ceria, lanthana, manganese, and precious metals canalso be added without further, high-temperature treatment.

In accordance with the method of this invention, a plurality of zirconfibers or other porous matrix material is mixed with ayttria-stabilized-zirconia powder, forming a fiber/powder mixture. Thefiber/powder mixture is formed into a porous YSZ layer and calcined.After calcination, the porous YSZ layer is impregnated with ametal-containing salt solution. In accordance with one preferredembodiment of this invention, the metal-containing salt solutioncomprises a nitrate salt of a metal selected from the group consistingof Cu, Ni and mixtures thereof.

In accordance with one preferred embodiment of this invention, thefiber/powder mixture is made into a slurry with glycerol and thenapplied to the anode side of the dense YSZ electrolyte of a solid oxidefuel cell. In accordance with another preferred embodiment, the powderedfibers are added to a tapecast which can be included as a layer in acomposite with a second layer which will give dense YSZ. The system isthen calcined at a suitable temperature, for example 1550° C., for twohours. After addition of the cathode to the cathode side of the YSZelectrolyte, the porous YSZ layer on the anode side is impregnated withaqueous solutions of Ni(NO₃)₂ or Cu(NO₃)₂ to bring the metal content ofthe anode to at least 35% by weight metal, after which the anode iscalcined at 950° C. for two hours. It is very important either to mix anormal YSZ powder (about 20% by weight) with the zircon fibers in theoriginal glycerol slurry, or to add ceria or YSZ to the porous YSZ layerso as to provide sufficient oxide in the anode for ionic conductivity.Ceria in accordance with one embodiment of this invention is added usingan aqueous solution of Ce(NO₃)₃ to the porous anode, after addition ofthe metal. After drying, the anode is again calcined to 900° C. to formthe oxide or ceria. In accordance with a particularly preferredembodiment of this invention, ceria constitutes in the range of about 5%to about 40% by weight of the porous YSZ layer.

Example

Yttria-stabilized zirconia (8% Y₂O₃, Tosoh TZ-8Y, denoted as YSZd) wasused for the fabrication of the electrolyte and conventionally preparedanode for a solid oxide fuel cell. The electrolyte wafers were formedfrom YSZd by tapecasting, followed by calcination to 1400° C. for twohours. The cathodes were formed from a 50% by weight physical mixture ofSr—LaMnO₃ and YSZd powders, pasted onto the electrolyte with glycerol,and then calcined at 1250° C. for two hours. The conventional, Ni-cermetanode was prepared using a 50% by weight physical mixture of NiO andYSZd, followed by calcination to 900° C. This Ni-cermet was pasted ontothe electrolyte using glycerol and calcined to 1400° C.

In accordance with the method of this invention for preparation of ananode for a solid oxide fuel cell, a porous YSZ layer was prepared fromphysical mixtures of zircon fibers (YSZ, 75% porosity, with less thanabout 0.3% Si, Zircar Products, Inc., denoted as YSZp) and YSZd. Thephysical mixture (denoted as YSZm) was pasted onto the electrolyte usingglycerol and calcined to 1550° C. for two hours. After addition of thecathode, the porous YSZ layer was impregnated with aqueous solutions ofCu(NO₃)₂ (Fisher Scientific) or Ni(NO₃)₂ (Aldrich) followed bycalcination at 950° C. for two hours, at a Cu (or Ni) content of 40% byweight.

A sample doped with ceria was prepared by adding ceria in an amount ofabout 5 to about 40 weight percent of the anode material, to the anodeby impregnation using Ce(NO₃)₃6H₂O (Aldrich) followed by calcination to950° C. The fraction of YSZd used in the anode and the metal content ofthe anode were varied.

Pt electrodes were attached to both anodes and cathodes using a Pt ink(Engelhard, A4338), followed by calcination at 950° C. for thirtyminutes. The cells were sealed into Al₂O₃ tubes using quartz powder inpolyvinyl solutions. They were then conditioned in H₂ for three to fourhours at 950° C. The performance of the cells was measured using flowingH₂ at 1 atmosphere at the anode, and the cathode was open to air.

SEM images were obtained using a JEOL 6300 microscope equipped with anX-ray analyzer for EDX analysis. Samples were deposited onto carbon tapeand coated with a gold film before analysis. X-ray powder diffractionpatterns were obtained with a Rigaku XRD diffractometer, using Cu Kαradiation (λ=1.541838 Å). The mean crystallite size ({overscore (d)}) ofYSZ particles was determined from XRD line-broadening measurements usingthe Scherrer equation.

FIG. 1 shows the performance for series of cells prepared with Cu-cermetanodes at 800° C. In this series, the Cu content was maintained atapproximately 40% by weight, but the fraction of non-porous YSZd wasvaried. Performance for the pure zircon fibers (neat YSZp) was poor,giving a maximum power density of only 5.1 mW/cm² and a maximum currentdensity of 35 mA/cm². Adding YSZd to the layer improved the performancesignificantly, with the best performance being achieved at about 25% byweight YSZd. The maximum power density for this cell was nearly 50mW/cm², with a maximum current density of 210 mA/cm². Increasing thefraction of YSZd in the layer led to poorer performance. The results inFIG. 1 demonstrate the importance of maintaining the proper structure ofthe YSZ in the anode as well as the possibility of a deleterious effectof Si in the zircon fibers.

FIGS. 2A, 2B, and 2C show SEM pictures of several representative samplesof anodes taken for the purpose of investigating their morphologies.FIG. 2A is a micrograph of the conventional Ni-cermet taken at amagnification of 5000×. Shown is a dense film made up of about 1-micronparticles. Before exposure to H₂ (the fuel atmosphere), EDX analysis andXRD patterns show the presence of NiO particles as a physical mixturewith the YSZ ({overscore (d)}=28.5 nm). After exposure to H₂ at950°-800° C. (reducing atmosphere), NiO is reduced to Ni metal({overscore (d)}=26 nm), determining a small porosity (about 20%) to thecompact NiO-YSZ material. The micrographs of the neat zircon fibers,shown in FIG. 2B at 1500× after heating to 1550° C., show rods, roughly20 microns long and 5 microns in diameter. The film remains highlyporous, about 70% void, but contact between the rods appears to bepoorer. Finally, the film formed by adding 25% by weight YSZd and Cu inaccordance with the method of this invention is shown in FIG. 2C. Thestructure remains open due to the rod-like fibers. Even with theaddition of non-porous YSZd and significant amounts of Cu, the filmremains highly porous. Before H₂ exposure, small crystallites of CuO({overscore (d)}=12.3 nm) were formed on the YSZm material. After H₂exposure at 950° C., Cu metal particles ({overscore (d)}=34 nm) areformed, as was observed in the Ni-cermet case.

The effect of changing the Cu content in the anode for the YSZm madefrom 25% by weight YSZd is shown in FIG. 3. As can be seen, there is adefinite improvement in cell performance observed with increases in Cufrom about 20% to about 50% by weight Cu.

FIG. 4 shows a comparison of results for a Ni-cermet anode prepared byconventional methods, a Ni-cermet with 40% Ni prepared from YSZm inaccordance with one embodiment of this invention and a Cu-cermet with40% by weight Cu prepared from YSZm in accordance with one embodiment ofthis invention. The results for all three cells are virtually identical,with maximum power densities between 45 and 50 mW/cm². The similarity inperformance suggests that the performance of these three cells islimited by the electrolyte and cathodes and not the anodes. In addition,because the catalytic properties of Ni and Cu are very different, withH₂ dissociation occurring much more readily on Ni, this suggests thatthe catalytic properties of the metals are not crucial in thisapplication with H₂ fuels. Rather Ni and Cu are primarily electronicconductors in this case.

FIGS. 5 and 6 show the typical doping effect of ceria on Cu/YSZp andCu/YSZm prepared using the method of this invention. The data for thesecells shows that the power densities increased significantly with theaddition of ceria. For 40% by weight ceria addition to Cu/YSZp andCu/YSZm, the best results achieved were 151.2 mW/cm² and 146.4 mW/cm²,which compared to 5.1 mW/cm² and 46.6 mW/cm² obtained in the same cellswithout ceria. The measured current density on CeO₂/Cu/YSZp was muchhigher than neat Cu/YSZp according to ceria contents. However, thecurrent density does not increase over 20% by weight ceria on Cu/YSZm.In such ceria content range, there is a limit to the improvement of cellperformance obtained.

The effect of adding ceria and Pd, using dry methane as a fuel is shownin FIGS. 7 and 8. Before discussing the I-V curves, it is important toaddress the stability issues. As expected, the Ni-YSZ anode prepared inaccordance with the method of this invention deactivated rapidly. Whilewe did observe currents at 800° C., they decreased rapidly over theperiod of a few minutes with the formation of a carbonaceous residue.This is shown in FIG. 7, which shows the current density for aNi/CeO₂/YSZ (open circles) at a cell voltage of 0.5 volts with switchingof fuels from dry H₂ to dry CH₄ and back. The cell with a Ni/CeO₂/YSZanode deactivated rapidly. Visual inspection showed the presence ofcarbon. By contrast, the Cu/CeO₂/YSZ cells were entirely stable.Following exposure to dry CH₄ for up to three days, we observed noevidence of decreased performance or carbon formation.

The performance of the Cu-based cells with methane is shown in FIG. 8.For the CU-YSZ anode, the OCV was only about 0.5 volts and the powerdensity was minimal. Addition of ceria led to a dramatic improvement, asdid the addition of Pd. However, the maximum power density with methaneis still much lower than with hydrogen, about 80 mW/cm² compared to 165mW/cm². This difference between methane and hydrogen is lowered byworking at higher temperatures. At 900° C., the maximum power densityachieved with this cell was about 230 mW/cm² with H₂ and 160 mW/cm² withCH₄. This suggests that the catalytic reaction is at least partlylimiting the reaction with methane, but that reasonable performance canbe achieved with dry methane if a catalytic component is added.

As demonstrated hereinabove, we have developed an entirely new methodfor fabricating anodes which allows the addition of metals and/orcatalytic materials after the high-temperature calcination step. We havedemonstrated that we can achieve similar performance levels withCu-cermets as can be achieved with Ni-cermets. And, we have found thatceria plays an important role in anode design.

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof, and many detailshave been set forth for purpose of illustration, it will be apparent tothose skilled in the art that the invention is susceptible to additionalembodiments and that certain of the details described herein can bevaried considerably without departing from the basic principles of theinvention.

We claim:
 1. In a solid oxide fuel cell comprising an anode electrode, acathode electrode and a dense electrolyte disposed between said anodeelectrode and said cathode electrode, the improvement comprising: saidanode electrode comprising a porous YSZ layer having a plurality ofpores defined by an inner pore wall, said inner pore wall of at least aportion of said plurality of pores coated with an electron-conductingmetal having an oxide form which melts at a temperature less than about1550° C.; and said dense electrolyte comprising an ionically conductivematerial, whereby said dense electrolyte is ionically conductive but notelectronically conductive.
 2. A solid oxide fuel cell in accordance withclaim 1, wherein said electron-conducting metal is Cu.
 3. A solid oxidefuel cell in accordance with claim 1, wherein said anode electrodefurther comprises ceria.
 4. A solid oxide fuel cell in accordance withclaim 3, wherein a ceria content of said anode electrode is in a rangeof about 5% to 40% by weight of said porous YSZ layer.
 5. A solid oxidefuel cell in accordance with claim 1, wherein a metal content of saidanode electrode is at least about 35% by weight of said porous YSZlayer.
 6. A method for generating electricity comprising the steps of:introducing a dry hydrocarbon directly into an anode side of a solidoxide fuel cell comprising an anode electrode comprising a porous YSZlayer having a plurality of pores defined by an inner pore wall, saidinner pore wall of at least a portion of said plurality of pores coatedwith an electron-conducting metal having an oxide form which melts at atemperature less than about 1550° C., a cathode electrode, and anionically conductive, non-electronically conductive electrolyte disposedbetween said anode electrode and said cathode electrode; introducing anoxidant into a cathode side of said solid oxide fuel cell; and directlyoxidizing said dry hydrocarbon in said solid oxide fuel cell, resultingin generation of electricity.
 7. A method in accordance with claim 6,wherein said hydrocarbon comprises at least two carbon atoms.
 8. Amethod in accordance with claim 6, wherein said electron-conductingmetal is Cu.
 9. A method in accordance with claim 6, wherein said anodeelectrode further comprises ceria.
 10. In a solid oxide fuel cellcomprising an anode electrode, a cathode electrode and an electrolytedisposed between said anode electrode and said cathode electrode, theimprovement comprising: said anode electrode comprising a porous YSZlayer having a plurality of pores defined by an inner pore wall, saidinner pore wall of at least a portion of said plurality of pores coatedwith an electron-conducting metal having an oxide form which melts at atemperature less than about 1550° C.; and said electrolyte comprising anionically conductive dense material, whereby said electrolyte isionically conductive but not electronically conductive.
 11. A solidoxide fuel cell in accordance with claim 10, wherein saidelectron-conducting metal is Cu.